Potential Trump administration responses being discussed include expanding U.S. drone strikes, redirecting or withholding some aid to Pakistan and perhaps eventually downgrading Pakistan's status as a major non-NATO ally, the officials said, speaking on condition of anonymity.

Some U.S. officials, however, are sceptical of the prospects for success, arguing that years of previous U.S. efforts to curb Pakistan's support for militant groups have failed, and that already strengthening U.S. ties to India, Pakistan's arch-enemy, undermine chances of a breakthrough with Islamabad.

U.S. officials say they seek greater cooperation with Pakistan, not a rupture in ties, once the administration finishes a regional review of the strategy guiding the 16-year-old war in Afghanistan.

Precise actions have yet to be decided.

The White House and Pentagon declined to comment on the review before its completion. Pakistan's embassy in Washington did not immediately respond to a request for comment.

"The United States and Pakistan continue to partner on a range of national security issues," Pentagon spokesman Adam Stump said.

But the discussions alone suggest a shift toward a more assertive approach to address safe havens in Pakistan that have been blamed for in part helping turn Afghanistan's war into an intractable conflict.

Experts on America's longest war argue that militant safe havens in Pakistan have allowed Taliban-linked insurgents a place to plot deadly strikes in Afghanistan and regroup after ground offensives.

Although long mindful of Pakistan, the Trump administration in recent weeks has put more emphasis on the relationship with Islamabad in discussions as it hammers out a the regional strategy to be presented to Trump by mid-July, nearly six months after he took office, one official said.

"We've never really fully articulated what our strategy towards Pakistan is. The strategy will more clearly say what we want from Pakistan specifically," the U.S. official said, speaking on condition of anonymity.

Other U.S. officials warn of divisions within the government about the right approach and question whether any mix of carrots and sticks can get Islamabad to change its behaviour. At the end of the day, Washington needs a partner, even if an imperfect one, in nuclear-armed Pakistan, they say.

The United States is again poised to deploy thousands more troops in Afghanistan, an acknowledgment that U.S.-backed forces are not winning and Taliban militants are resurgent.

Without more pressure on militants within Pakistan who target Afghanistan, experts say additional U.S. troop deployments will fail to meet their ultimate objective: to pressure the Taliban to eventually negotiate peace.

"I believe there will be a much harder U.S. line on Pakistan going forward than there has been in the past," Hamdullah Mohib, the Afghan ambassador to the United States, told Reuters, without citing specific measures under review.

Kabul has long been critical of Pakistan's role in Afghanistan.

Pakistan fiercely denies allowing any militants safe haven on its territory. It bristles at U.S. claims that Pakistan's spy agency, the Inter-Services Intelligence (ISI) directorate, has ties to Haqqani network militants blamed for some of the deadliest attacks in Afghanistan.

"What Pakistan says is that we are already doing a lot and that our plate is already full," a senior Pakistani government source told Reuters, speaking on condition of anonymity.

The source doubted the Trump administration would press too hard, saying: "They don’t want to push Pakistan to abandon their war against terrorism."

Pakistani officials point towards the toll militancy has taken on the country. Since 2003, almost 22,000 civilians and nearly 7,000 Pakistani security forces have been killed as a result of militancy, according to the South Asia Terrorism Portal, which tracks violence.

Experts say Pakistan's policy towards Afghanistan is also driven in part by fears that India will gain influence in Afghanistan.

IS PAKISTAN AN ALLY?

Nuclear-armed Pakistan won the status as a major non-NATO ally in 2004 from the George Bush administration, in what was at the time seen in part as recognition of its importance in the U.S. battle against al Qaeda and Taliban insurgents.

The status is mainly symbolic, allowing limited benefits such as giving Pakistan faster access to surplus U.S. military hardware.

Some U.S. officials and experts on the region scoff at the title.

"Pakistan is not an ally. It’s not North Korea or Iran. But it’s not an ally," said Bruce Riedel, a Pakistan expert at the Brookings Institution.

But yanking the title would be seen by Pakistan as a major blow.

"The Pakistanis would take that very seriously because it would be a slap at their honour," said a former U.S. official, speaking on condition of anonymity.

Lisa Curtis, senior director for South and Central Asia at the National Security Council, co-authored a report with Husain Haqqani, Pakistan's former ambassador to Washington, in which they recommended the Trump administration warn Pakistan the status could be revoked in six months.

"Thinking of Pakistan as an ally will continue to create problems for the next administration as it did for the last one," said the February report.

It was unclear how seriously the Trump administration was considering the proposal.

The growing danger to Afghanistan from suspected Pakistan-based militants was underscored by a devastating May 31 truck bomb that killed more than 80 people and wounded 460 in Afghanistan's capital, Kabul.

Afghanistan's main intelligence agency said the attack - one of the deadliest in memory in Kabul - had been carried out by the Haqqani network with assistance from Pakistan, a charge Islamabad denies.

Washington believes the strikes appeared to be the work of the Haqqani network, U.S. officials told Reuters.

U.S. frustration over the Haqqani's presence in Pakistan has been building for years. The United States designated the Haqqani network as a terrorist organisation in 2012. U.S. Navy Admiral Mike Mullen, then the top U.S. military officer, told Congress in 2011 that the Haqqani network was a "veritable arm" of the ISI.

The potential U.S. pivot to a more assertive approach would be sharply different than the approach taken at the start of the Obama administration, when U.S. officials sought to court Pakistani leaders, including Army chief General Ashfaq Kayani.

David Sedney, who served as Obama's deputy assistant secretary of defence for Afghanistan, Pakistan and Central Asia from 2009 to 2013, said the attempt to turn Islamabad into a strategic partner was a "disaster."

"It didn't affect Pakistan's behaviour one bit. In fact, I would argue it made Pakistan's behaviour worse," Sedney said.

MORE DRONES, CASH CUT-OFF

Pakistan has received more than $33 billion in U.S. assistance since 2002, including more than $14 billion in so-called Coalition Support Funds (CSF), a U.S. Defense Department programme to reimburse allies that have incurred costs in supporting counter-insurgency operations.

It is an important form of foreign currency for the nuclear-armed country and one that is getting particularly close scrutiny during the Trump administration review.

Last year, the Pentagon decided not to pay Pakistan $300 million in CSF funding after then-U.S. Secretary of Defense Ash Carter declined to sign authorization that Pakistan was taking adequate action against the Haqqani network.

U.S. officials said the Trump administration was discussing withholding at least some assistance to Pakistan.

Curtis' report also singled out the aid as a target.

But U.S. aid cuts could cede even more influence to China, which already has committed nearly $60 billion in investments in Pakistan.

Another option under review is broadening a drone campaign to penetrate deeper into Pakistan to target Haqqani fighters and other militants blamed for attacks in Afghanistan, U.S. officials and a Pakistan expert said.

"Now the Americans (will be) saying, you aren't taking out our enemies, so therefore we are taking them out ourselves," the Pakistan expert, who declined to be identified, said.

Pakistan's army chief of staff last week criticized "unilateral actions" such as drone strikes as "counterproductive and against (the) spirit of ongoing cooperation and intelligence sharing being diligently undertaken by Pakistan".